How dependency injection works in Drupal?

In Drupal, dependency injection is implemented using the Symfony Dependency Injection component, which is a popular PHP library for managing dependencies in object-oriented applications. Drupal is built on top of Symfony framework and leverages its components for various functionalities, including dependency injection.

Here's a high-level overview of how dependency injection is implemented in Drupal:

1. Service Container: Drupal has a service container, which is a centralized registry for managing dependencies. It is responsible for creating and managing objects (services) that are used throughout the Drupal system.

2. Service Definitions: Services in Drupal are defined in YAML files, known as service definitions, which specify the class that should be instantiated, along with its dependencies and other configuration options.

3. Service Tags: Drupal services can also be tagged with metadata, allowing them to be grouped and processed together. For example, all services that implement a certain interface can be tagged with the same tag, making it easy to find and use them.

4. Automatic Service Wiring: Drupal automatically wires services together based on their dependencies defined in the service definitions. When a service is requested from the service container, Drupal automatically resolves its dependencies and injects them into the service.

5. Dependency Injection in Code: Services can be injected into other services or classes using type hinting in the constructor, setter methods, or using the container parameter in functions. Drupal's container is available as a global variable $container in Drupal 8/9, or can be injected using the ContainerInterface in Drupal 9/10.

6. Service Overrides: Drupal allows service definitions to be overridden in custom modules, themes, or profiles, providing a way to customize the behavior of services without modifying core or contributed modules. This is done by implementing a Service Provider which extends the ServiceProviderBase class. The name of the Service Provider should be prepended by the module name in upperCamelCase. For example: MyModuleServiceProvider.php

7. Service Subscribers: Drupal allows service subscribers to define a list of services they are interested in, and Drupal's service container will automatically provide those services to them. This allows for automatic dependency injection without having to explicitly inject each service.

8. Plugin System: Drupal's plugin system also uses dependency injection. Plugins define their dependencies in their annotations, and Drupal's plugin manager automatically resolves and injects those dependencies when plugins are instantiated.

Overall, Drupal's implementation of dependency injection using the Symfony Dependency Injection component provides a powerful and flexible way to manage dependencies and promote modular and testable code in Drupal applications.